Integrated electrical circuits and other microstructured components are conventionally produced by applying a plurality of structured layers onto a suitable substrate which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet (DUV) spectral range. The wafer coated in this way is subsequently exposed in a projection exposure apparatus. A pattern of diffracting structures, which is contained in a mask, is thereby imaged onto the photoresist with the aid of a projection objective.
After the photoresist has been developed, the wafer is subjected to an etching process so that the top layer becomes structured according to the pattern on the mask. This process is repeated until all the layers have been applied on the wafer.
One of the main aims in the development of microlithographic projection exposure apparatus is to be able to generate structures with smaller and smaller dimensions on the wafer, so as to increase the integration density of the components to be produced. By employing a wide variety of measures, structures whose dimensions are less than the wavelength of the projection light being used can now be generated on the wafer.
One of these measures is to introduce an immersion liquid into an intermediate space between the projection objective and the wafer. This makes it possible, for example, to use projection objectives with a particularly high numerical aperture which may significantly exceed 1.
However, one effect of decreasing the structure size on the wafer is that the structures on the mask to be imaged also become smaller and smaller. Reduction of the imaging scale of the projection objective is generally avoided on cost grounds, since it may entail very wide-ranging technical changes in the design of the projection objectives. If the structure widths of the transparent structures contained in the mask are of the order of the wavelength of the projection light being used, or even significantly less, then effects that undesirably impair the optical imaging by the projection objective may occur in amplitude masks and most phase masks. In general, only masks which contain no opaque or light-attenuating structures do not exhibit these effects. The term amplitude masks refers to those masks in which—unlike in pure phase masks—it is not the phase but the amplitude of light passing through which is influenced. The opaque or light-attenuating structures are often structured layers of Cr or MoSi.
One of the aforementioned undesired effects, which occur with very small structure sizes in such masks, is that the mask has a polarizing effect on the projection light passing through. The most significant effect in this case can be that projection light whose polarization direction is aligned parallel with the longitudinal dimension of the structures will be transmitted better by these structures than projection light with a polarization direction perpendicular thereto. The effect of this polarization dependence of the transmissivity is that structures which are equivalent as such but are oriented differently will be imaged with different intensities on the photoresist. Owing to the sharp exposure threshold of the photoresist, variations in the quantity of light energy impinging per unit area (often referred to as exposure dose) can have a direct effect on the width of the structures lithographically generated on the wafer. Such a dependence of the structure widths on the orientation of the structures contained in the mask is generally undesirable.
Another polarizing effect of narrow structures, however, may be that projection light with a polarization aligned parallel with the longitudinal dimension of the structures propagates with a different velocity in the transparent structures than projection light with a polarization aligned perpendicularly thereto. This can lead to phase differences between mutually perpendicular polarization components, as similarly occur in retardation plates. Such phase differences are generally undesirable in masks, since they also cause the structure widths on the wafer to be dependent on the orientation of the structures contained in the mask. One reason for this is that certain optical elements in projection objectives, for example polarization-selective beam splitting layers, have a polarization-dependent transmissivity or reflectivity. In this way, for example, the conversion of linearly polarized light into elliptically polarized light in the mask may also affect the intensities and therefore the structure widths on the wafer.
Since the aforementioned birefringence of narrow structures also depends on the distances between adjacent structures, more densely arranged structures having identical widths will typically modify the state of polarization in a different manner than structures spaced apart by wider distances.